Lysosomes are the main components of a eukaryotic digestive
system responsible for degradation of both extracellular
and intracellular materials. The low internal pH of these
organelles (about pH 5) provides an acidic environment
suited to the operation of lysosomal hydrolytic enzymes
(1-3). Studies of the isolated organelle indicate that its low
internal pH is sustained by an ATP-driven H+ pump (4-9),
which operates most effectively in the presence of permeant
anions, in either an electrogenic (7, 8) or an electroneutral (9)
fashion. This lysosomal H+ pump may be distinguished from
other ion-motive pumps by its characteristic response to
several inhibitors (7, 8). Lysosomal acidification is insensitive
to both oligomycin and orthovanadate, inhibitors that
block mitochondrial FoF1 and plasma membrane E1E2-
phosphoenzyme ATPases, respectively, whereas levels of
N-ethylmaleimide (MalNEt) that have no effect on either
FoF1 or E1E2 pumps markedly reduce lysosomal acidification.
In its dependence on an external anion and in its pharmacological
profile, the lysosomal H+ pump resembles ATPdriven
acidification mechanisms associated with the eukaryote
vacuolar membrane system-endoplasmic reticulum
(10), Golgi membranes (11), vesicles mediating both endoand
exocytosis (12-17), and vacuoles (18-20). These minor
organelles serve widely divergent functions, yet a superficial
characterization suggests important similarities with respect
to their resident H+ pumps. To determine whether this
homology is retained at a more detailed level of analysis, we
have examined the lysosomal H+ pump in a reconstituted
state, with the specific goal of answering questions related to
ionic selectivity.
Lysosomes are the main components of a eukaryotic digestivesystem responsible for degradation of both extracellularand intracellular materials. The low internal pH of theseorganelles (about pH 5) provides an acidic environmentsuited to the operation of lysosomal hydrolytic enzymes(1-3). Studies of the isolated organelle indicate that its lowinternal pH is sustained by an ATP-driven H+ pump (4-9),which operates most effectively in the presence of permeantanions, in either an electrogenic (7, 8) or an electroneutral (9)fashion. This lysosomal H+ pump may be distinguished fromother ion-motive pumps by its characteristic response toseveral inhibitors (7, 8). Lysosomal acidification is insensitiveto both oligomycin and orthovanadate, inhibitors thatblock mitochondrial FoF1 and plasma membrane E1E2-phosphoenzyme ATPases, respectively, whereas levels ofN-ethylmaleimide (MalNEt) that have no effect on eitherFoF1 or E1E2 pumps markedly reduce lysosomal acidification.In its dependence on an external anion and in its pharmacologicalprofile, the lysosomal H+ pump resembles ATPdrivenacidification mechanisms associated with the eukaryotevacuolar membrane system-endoplasmic reticulum(10), Golgi membranes (11), vesicles mediating both endoandexocytosis (12-17), and vacuoles (18-20). These minororganelles serve widely divergent functions, yet a superficialcharacterization suggests important similarities with respectto their resident H+ pumps. To determine whether thishomology is retained at a more detailed level of analysis, wehave examined the lysosomal H+ pump in a reconstitutedstate, with the specific goal of answering questions related toionic selectivity.
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